The present invention relates generally to microfluidic control techniques. In particular, the present invention provides a method of plasmon assisted optofluidics using a laser. More particularly, the present invention provides a method for optically controlling fluid in a microchannel using a plasmon resonance in fixed arrays of nanoscale metal structures to produce localized evaporation of the fluid when illuminated by a stationary, low power laser. Merely by way of example, the invention has been applied to drag the surface of the fluid, drive evaporative pumping, and provide intra-channel distillation and sample concentration, but it would be recognized that the invention has a much broader range of applicability.
Current microfluidics is realized through pumping, which is an excellent means for transport, mixing, and metering. Ideally, this and other complex functionalities would occur directly on-chip. However, the majority of microfluidic systems employ off-chip, mechanical pumps combined with valve networks to direct fluid flow. Electro-kinetic transport can be more compact and flexible, but it depends on liquid conductivity and requires large voltages and a fabrication method that integrates the fluidic and electronic circuitry. Electrowetting based devices have great utility, but are most naturally limited to discrete, droplet based devices.
Recently there has been increased interest in using optical transport methods for microfluidics. This approach uses optical beams to induce flow without connected pumps or electrical circuitry. An example is photothermal transport by resonant heating of nanoparticles in solution, which can be used to control the position of the free surface of a fluid along a complex circuit without the need for valves. Although it can be arbitrarily applied anywhere on a chip, however, this method requires that the optical beam be translated to transport the fluid. Furthermore, it may not be desirable or possible to have nanoparticles freely suspended in liquid solution, because the changing concentration of the suspended nanoparticles makes difficult for controlling the flow rate for a given laser power.
Another aspect regarding the fluid pumping in a microchannel involves interphase mass transfer. A conventional method uses a series of heaters, which are typically embedded in the channel, to produce a vapor bubble as well as a thermal gradient between the two ends of the bubble. Mass-transfer occurs as fluid on the warmer interface is vaporized and then condensed on the cooler side. In addition to pumping, vapor mass-transfer provides a simple means to separate both soluble and insoluble components of a mixture. However, although it can be applied on-chip, this method requires the high temperatures to create and to prevent the collapse of the vapor bubble and precludes many applications, especially biological ones.
From above, it is seen that there is a need in the art for an improved method and system for controlling fluid in a microchannel structure with on-chip functionality for pumping, distillation, and sample concentration based on ambient temperature interphase mass-transfer.